1. Field of the Invention
The present invention relates to novel composite polyester films and the production thereof, and, more especially, to novel biaxially drawn, thin composite polyester films having on at least one outer face surface thereof a novel and original roughness comprising protuberances in the shape of "domes" and "peaks" which makes it possible to obtain a highly satisfactory characteristics (ease of winding at high speed or slip) and abrasion phenomena (resistance to white powder formation). The subject novel films are well adapted for the manufacture of magnetic recording tapes having good electromagnetic properties.
2. Description of the Prior Art
Already known to this art are thin composite polyester films capable of being used, in particular, for the manufacture of audio or video recording tapes and incorporating a transparent layer, typically unfilled or, if appropriate, very lightly filled, and at least one outer layer incorporating an additive consisting of a fine filler and making it possible to confer the slip properties which are required for good machinability.
Such biaxially drawn composite films are, for example, described in British Pat. No. 1,397,653 to La Cellophane, and include, as a core element, a layer of transparent (unfilled) polyester, and on the outside at least one less transparent and more slippery polyester layer containing solid particles of a size below 2 microns and originating from the catalyst residue.
With such composite films it is not an easy matter to adjust the roughness to the intended application by modifying the microrelief and hence the size, the number and the distribution of the protuberances. It also is not an easy matter to control the difference in roughness between the two outer face surfaces of the composite film.
It is difficult, furthermore, to altogether avoid the presence of peaks whose size is too great relative to all of the other protuberances (it is well known that this presence is undesirable, since it implies, in fact, information losses and hence faults such as "drop out" in the case of video magnetic tapes, and the like).
Also known, according to European Pat. No. 35,835, are composite films consisting of an unfilled oriented crystalline film and a filled and heat-sealable amorphous film; the thickness of the amorphous layer is lower than that of the median diameter of the filler particles present in this layer.
In sum, it is therefore difficult to conveniently adjust the microroughness of the outer face surface of the film (or of both outer face surfaces), while at the same time eliminating the interfering protuberances.